In an electrophotographic modular printing machine of known type, for example, the NexPress 2100 printer manufactured by NexPress Solutions, Inc., of Rochester, N.Y., color toner images are made sequentially in a plurality of color imaging modules arranged in tandem, and the toner images are successively electrostatically transferred in registration to a surface that is moved past the imaging modules. This transfer can be made to receiver member that is moved past the imaging modules or to an intermediate transfer member that receives all of the toner images to be used in an image and then transfers these to a receiver member that is moved through a transfer nip. After all of the toner images have been transferred to the receiver member, the receiver member is fused.
As is known, when the color toners are deposited one upon the other, the respective color toners form toner stacks that will create particular colors at particular locations of the image formed on the receiver member after fusing. The height of a respective color toner stack is the sum of the toner contributions of each color of toner applied at a particular location. FIG. 1 depicts an exemplary section of a receiver member 2 having a plurality of color toner stacks 4A-4N before a fusing operation. As can be seen from FIG. 1, color toner stacks 4A-4N provide a range of color toner stack heights before fusing, with the toner stack heights varying based upon the amount of a particular color toner applied thereto.
FIG. 2 shows the section of FIG. 1 after fusing. As can be seen in FIG. 2, color toner stacks 4A-4N typically flatten to form a toner mass 6 because of the pressure and heat applied during fusing. However, relief differences remain on upper surface 8 of toner mass 6 between, for example, an area 10 that corresponds to high density color image elements shown in FIG. 1 as having higher toner stack heights e.g. toner stack 4D and an area 12 that corresponds to lower density color image elements shown in FIG. 1 as having a lower toner stack height e.g. toner stack 4E in FIG. 1. These variations are particularly noticeable in that they disrupt the extent to which surface 8 of a toner printed image reflects light in a specular manner. The capability of a printed image to reflect incident light in a specular fashion is typically referred to as gloss.
In a fused toner image, several factors impact gloss. The primary factors indicate the refractive index of the fused toner and the surface roughness of the fused toner. It will be appreciated that more uniform gloss can therefore be provided on an image by forming a toner image with an upper most surface having less surface roughness.
Electrostatographic printers having a three, four, or more color (multicolor) capability are known to also provide an additional toner depositing assembly for depositing clear toner. U.S. Pat. No. 5,234,783, issued on Aug. 10, 1993, in the name of Yee S. Ng, describes a process where gloss of a printed image is improved by applying gloss improving clear toner image to the color toner stacks forming the image. The gloss producing clear toner image varies inversely according to the expected stack heights provided by the other images providing ultimately an even height toner image. Similarly, U.S. Pat. No. 7,016,621, issued on Mar. 21, 2006 in the name of Yee S. Ng, describes the formation of a toner image wherein back-transfer artifacts are reduced or eliminated without the need or expense of providing uniform coverage of clear toner to the print wherein a five color tandem printer is used to print fewer than five colors. The fifth station may be used during the one pass through the printer apparatus, as a clear toner station, to deposit relatively less clear toner in relatively higher colored areas and relatively more clear toner in areas having relatively lower amounts of colored toner.
Such gloss improving clear toner images are also known in the art and referred to as inverse mask toner images. As is noted in the '783 patent, inverse mask toner images can be recorded, for example, on top of the color toner stacks or beneath the color toner stacks.
Methods for determining the inverse mask, however, have remained computationally intense in that, in general, an amount of clear toner to be laid down is calculated for each pixel location in the toner image formed by the multi-layer toner image. See for example, commonly assigned U.S. Pat. No. 7,236,734, entitled Method and Apparatus for Electrostatographic Printing With Enhanced Color Gamut, issued to Ng. et al. on Jun. 26, 2007. As is described therein, incoming image data to be printed is input to a Raster Image Processor and converted to printer dependent color separation image data in each of the four-color images printed by the printer apparatus. The clear toner image generator, which also may be a part of the RIP, creates a clear toner “image” from the four color separation images previously created assuming that glossing is to be done and an inverse mask is to be established for printing of the clear toner.
It is further noted in the '734 patent, that, as a convenience in calculation, rather than determining pigmented toner coverage at any pixel area in accordance with the sum of the four color contributions at that pixel location, one may select the maximum pixel percent contribution by a color separation at that pixel location as the percentage of pigmented toner coverage present at that location for use in determining the amount of clear toner overcoat to be applied in the inverse mask. The use of the single color that is maximum at that location in conjunction with the particular selected inverse mask curve's roll off starting at the mid-tone helps ensure that total toner coverage of the four colors plus clear toner at the pixel location is below 320%, and this is basically true for the entire color gamut. As a further convenience in calculation, in lieu of making such calculation for the inverse mask using a pixel by pixel calculation, one may group local areas of say 4.times.4 pixels or 16 pixels to determine the amount of clear toner in the inverse mask calculation for this small area formed by a group of pixels.
Accordingly, using such techniques, the process of determining an inverse mask is performed after a raster imaging process performs color separation on the supplied image.
Further, to the extent that an image data is submitted for printing in a format that is not readily processed by conventional raster image processors, it may further be necessary to convert digital image data supplied for printing into a format that is preferred by the Raster Image Processor. For example, it may necessary to convert submitted image data that is organized according to one color model into image data that is organized according to a different model. In such situations, it can be necessary to first convert the submitted image data into a data format that can be readily processed by the Raster Image Processor. Accordingly, in such a situation, two conversion steps can be required before the inverse mask image can be generated.
In most applications such conversions can be executed in a timely, technically, commercially, and economically feasible manner.
However, in some circumstances, for example, where images are being printed that incorporate variable data that can change from print to print, there can be very little time available to process the image data before it is used for printing. In such circumstances, it can be beneficial to have a printer and method that enable the creation of a gloss improving inverse mask without requiring generation of a toner image for each color non-masking toner layer.
Accordingly, what are needed are new printers and methods for determining an inverse mask toner image.